Metal strips are usually coated, in order to be able to bring about desired functionalities, in particular on the surface of the metal strips. Depending on the type, nature and desired functionality of the surface, various methods of pretreatment are used. Common important desired aims of surface treatments are cleaning the surface to be coated, providing the surface with corrosion protection, optimum bonding conditions between the surface and the subsequent coating and also for example providing the surface with some other functional coating.
Increasing requirements for the metal strips, including with regard to their mechanical and thermal stability, for example during forming processes involving great deformation loading, require very high quality of the coatings with regard to their bonding on the treated surfaces. For this reason, it is required for example that pretreatment layers have a uniform and uninterrupted application, in order to ensure continuous and good bonding for subsequent coatings of the surface. Inspection of the coated surfaces is of great importance in this connection, to allow coating defects to be ruled out, or largely ruled out.
A known method for inspecting the quality of applied coatings, for example coatings applied by means of roller application, is analytics of the coated surfaces by means of infrared radiation for NIR analytics, NIR being the abbreviation for a wavelength range of the infrared radiation that is used. However, the measurements that are typically performed with such methods have the disadvantage that, with different coatings, great differences in the infrared spectra are obtained, whereby the evaluation of the infrared spectra requires great effort. A further disadvantage of NIR analytics is often that a measurement of individual points is performed, for example a traversing point measurement. This entails the disadvantage that uninterrupted monitoring of the coating is not possible. This measuring method is consequently inadequate for application areas in which surfaces without coating defects or largely without coating defects are required.
An example of a method for coating a metal strip is presented in WO 2009/112452. The method explained comprises recording IR-spectroscopic measurement data on the basis of Fourier transform IR spectroscopy. Although continuous recording is possible during the coating of the metal strip, in the case of the method described it has proven to be disadvantageous on the one hand that there is a necessity inherent in the method for comparatively complex evaluation of the measured spectra, and on the other hand that the area coverage is limited by the chosen type of spectrometer.
A method for ascertaining defects in a surface of a moving material, in particular a metal strip, is disclosed by WO 2010/033113. The method explained comprises heating the metal strip with subsequent thermography. However, because of the required process of active heating, the method described is complex, and therefore does not allow uninterrupted checking of the surface quality of the coated surface, or only allows it with great effort.
Thus a need exists for a method for coating a surface of a metal strip where the method makes it possible for the surface quality of the coated surface to be checked uninterruptedly, preferably both with regard to the surface coverage and with regard to the time dimension.